Sinusitis is an inflammation of the nasal sinus cavities, which
are moist, hollow spaces in the bones of the skull.
The sinuses are located behind the eyebrows, cheekbones, and nose.
The sinuses help moisten and warm air that is filtered by the nose, serving to
protect the lungs.
The sinus cavities, nose, and lungs are lined with mucous membranes, which
protect the airways by trapping irritants that are inhaled. Tiny hair-like
filaments called cilia are in constant movement and sweep mucus and the trapped
irritants out of the airways and nasal passages.
In the sinuses, the mucus drains out of a tiny opening called the ostium.
The ostium of any sinus cavity can get plugged. When this blockage prevents the
flow of mucus, the pressure builds up, leading to sinusitis (inflammation and
pain). If the blocked sinus becomes infected with bacteria, the sinus becomes
even more inflamed and painful.

SymptomsNasal discharge is a symptom and some times it is a cause of sinusitis: